Last month, I introduced Conophytum burgeri—Burger’s Onion—and promised to follow up with some information on the cultivation of this South African desert oddity. I’ve been growing these plants successfully since my undergrad days, and while the Onion is not really an ideal subject for the mesemb newbie, it’s quite possible to grow and flower one on a windowsill, if certain conditions are met.
Burger’s Onion is first and foremost a sunlight pig. If you are going to attempt windowsill cultivation, make sure you can secure a spot in a south window that gets at least six hours of direct sun per day (an airy, well-lit greenhouse makes things so much easier…). In winter, I don’t think it would be physically possible for a C. burgeri plant to receive too much sun here in New England. In summer, a potted plant might have trouble if it cooked in direct sun all day, but shade can still be minimal. Ideally, the plants should spend winter in a place that gets quite cool at night, into the 50s or 40s.
Conophytum burgeri’s habit of hiding inside of the dried remains of its old leaves can lead to trouble. In the wild, I suspect that older leaf sheaths are weathered away about as fast as they accumulate, but under glass the annual layers of dead tissue just keep stacking up. Some growers apparently peel their plants entirely in the autumn, to expose the living leaves directly, but I feel that this creates an unnecessary risk of sunburn in the spring. When I have some time to kill, every year or two, I do like to pick away all but the innermost layer of sheathing on my plants, to let in light and air.
The watering regime for C. burgeri plants should be on the stingy side. For most cultivation situations, this means a light watering every week or so, when the soil is quite dry, during the winter growing period, and nothing but a bit of mist every few days during sunny weather in summer. If the plants get too much water, they puff up and split, and are likely to rot.
The soil mix for Burger’s Onion should be lean, with little or no organic matter: I use poor sandy-loam subsoil, liberally cut with course sand, perlite, pumice and/or Turface (fired clay granules). Mature plants get almost no fertilizer. A splash of quarter-strength Miracle Grow (or something similar) once a year is about as much as you can get away with without risking bloating, cracking, and rotting.
Conophytum burgeri plants can be obtained occasionally from Mesa Garden or Steven Hammer’s Sphaeroid Institute, and seed is usually available at Mesa or from the Mesemb Study Group. Growing from seed is rewarding, but slow: even under ideal conditions, it will take three or four years to get a plant the diameter of a dime. Germination is quick and easy for seed sown on the surface of moist soil in August or September, but the minute seedlings have to be coddled along for their first year of life. Water when the medium is just barely dry, and spray lightly with dilute fertilizer every few weeks. Otherwise, seedlings and adult plants prefer similar conditions: bright sun, cool nights, and plenty of fresh air.